1660s, "action of enlightening," from enlighten + -ment. Used only in figurative sense, of spiritual enlightenment, etc. Attested from 1865 as a translation of German Aufklärung, a name for the spirit of independent thought and rationalistic system of 18c. Continental philosophers.
For the philosophes, man was not a sinner, at least not by nature; human nature -- and this argument was subversive, in fact revolutionary, in their day -- is by origin good, or at least neutral. Despite the undeniable power of man's antisocial passions, therefore, the individual may hope for improvement through his own efforts -- through education, participation in politics, activity in behalf of reform, but not through prayer. [Peter Gay, "The Enlightenment"]